Longitudinal T Cell Responses against Ancestral, Delta, and Omicron SARS-CoV-2 Variants Determined by Rapid Cytokine Release Assay in Whole Blood.

T cell immunity to natural SARS-CoV-2 infection may be more robust and longer lived than Ab responses. Accurate assessment of T cell responses is critical for understanding the magnitude and longevity of immunity across patient cohorts, and against emerging variants. By establishing a simple, accurate, and rapid whole blood test, natural and vaccine-induced SARS-CoV-2 immunity was determined. Cytokine release in whole blood stimulated with peptides specific for SARS-CoV-2 was measured in donors with previous PCR-confirmed infection, suspected infection, or with no exposure history (n = 128), as well as in donors before and after vaccination (n = 32). Longitudinal assessment of T cell responses following initial vaccination and booster vaccination was also conducted (n = 50 and n = 62, respectively). Cytokines were measured by ELISA and multiplex array. IL-2 and IFN-γ were highly elevated in PCR-confirmed donors compared with history-negative controls, with median levels ∼33-fold and ∼48-fold higher, respectively. Receiver operating curves showed IL-2 as the superior biomarker (area under the curve = 0.9950). Following vaccination, all donors demonstrated a positive IL-2 response. Median IL-2 levels increased ∼32-fold from prevaccination to postvaccination in uninfected individuals. Longitudinal assessment revealed that T cell responses were stable up to 6 mo postvaccination. No significant differences in cytokine production were observed between stimulations with Wuhan, Delta, or Omicron peptides. This rapid, whole blood-based test can be used to make comparable longitudinal assessments of vaccine-induced T cell immunity across multiple cohorts and against variants of concern, thus aiding decisions on public health policies.